Countermeasure systems designed to make a target virtually invisible to an unfriendly tracking radar must include means for ascertaining the scanning frequency f.sub.s of that radar with a high degree of accuracy. Passive systems of this type merely serve to detect and measure the characteristic parameters of radars operating in the environment whereas active systems utilize that information for the purpose of sending out interfering signals effective to divert the radar beam from its target. Such active systems may be mounted, for example, on platforms aboard ships or aircrafts intended to elude detection.
The frequency f.sub.s of a conically scanning tracking radar generally lies in a range of roughly 30 to 300 Hz. In order to estimate its actual value, within tolerance limits of 1 Hz, a passive circuit arrangement may comprise a multiplicity of phase-locking loops (PLL) tuned to respective discrete frequencies within the aforementioned range. A circuit arrangement of this description, code-named "SERO", is used in the SELENIA INS-3 system of our assignee, Selenia Industrie Elettroniche Associate S.p.A.; in that system the lowest PLL frequency latched onto the detected radar signal is taken as the wanted scanning frequency f.sub.s in order to exclude its higher harmonics. Such a circuit arrangement limits the maximum RMS error, which varies with the measured frequency, to a value between 1 and 2 Hz.
Active systems of the so-called "inverse gain" type, such as those code-named SL/ALQ 234 and 238, emit an interfering signal whose frequency should match the carrier frequency of the radar signal, with a deviation less than the bandwidth of the servo system of the radar antenna, and which is modulated in amplitude with an envelope in phase opposition to that of the incident carrier. The carrier frequency of the emitted interfering signal is progressively varied until its disturbing effect upon the radar signal has resulted in a disalignment of the tracking beam from the target as determined from a significant increase in the depth of modulation of the detected signal. The rate of change in the emitted carrier frequency must be slow enough, generally on the order of 0.5 Hz/sec, to induce an appreciable pointing error in the operation of the tracking radar. However, the neutralization of that radar ought to be accomplished in a relatively short time, usually of about 10 seconds, so that the frequency sweep can cover a band of only a few Hz in the available interval.
Thus, the PLL circuitry referred to above can be used without any prior knowledge of the scanning frequency f.sub.s whereas the inverse-gain system will be effective only when the scanning frequency is known to lie within a relatively narrow band of about 4 to 5 Hz.